There are many instances when it is advantageous to use photographs or electronically stored images to aid a viewer in observing the features and spatial relationships of an object. The features the viewer would like to locate or see are sometimes obscured or are difficult to locate with the unaided eye.
One example is during a surgical procedure. A surgeon may take a photograph or an electronic image of an area before surgery begins. The image may be from previous diagnostic tests such as magnetic resonance imaging (MRI) or computerized axial tomography (CAT). The surgeon will refer to this image during surgery as a guide for the procedure. For example, an image may be taken of a tumor before starting a removal procedure. During the procedure, the surgeon can refer back to the image to assist in locating the tumor or to ensure that the entire tumor has been removed.